In a construction machine, e.g., a hydraulic excavator, a front operating mechanism including a boom, an arm, a bucket, etc. and an upper swing body are operated by hydraulic actuators, e.g., a hydraulic cylinder and a hydraulic motor. Those hydraulic actuators are operated by a hydraulic fluid delivered from a hydraulic pump which is driven by an engine. The upper swing body is covered with a cover, and the engine and the hydraulic pump are disposed in an engine room formed within the cover. In that type of construction machine, it is usual that, for the purpose of cooling the engine, a cooling fan disposed in the engine room is driven to introduce open air through intake holes formed in the cover, thereby producing cooling air. As the cooling fan, the so-called axial fan (propeller fan) rotated by a driving force from an engine crankshaft is used in many cases. The cooling air produced by the cooling fan is introduced into the engine room and passes through various heat exchangers for cooling them, and is then discharged to the exterior of the engine room through discharge holes formed in the cover. The heat exchangers include, for example, an intercooler for cooling compressed air pressurized by a turbocharger which is mounted on the engine, a radiator for cooling engine cooling water, and an oil cooler for cooling working oil in a hydraulic driving system.
In the above-described cooling fan directly driven by the engine, the rotation speed of the cooling fan is proportional to the engine revolution speed. Therefore, it may occur sometimes that the cooling water for the radiator and the working oil for the oil cooler are overcooled and a longer time is taken for warm-up operation. To avoid such a drawback, a system for driving the cooling fan independently of the engine revolution has hitherto been proposed, for example, which comprises a cooling fan for forcibly cooling a radiator and an oil cooler, a fan hydraulic motor for driving the cooling fan, a variable-displacement fan hydraulic pump capable of controlling the rotation speed of the fan hydraulic motor, a cooling water temperature sensor for detecting the temperature of cooling water, a working oil temperature sensor for detecting the temperature of working oil, an engine revolution speed sensor for detecting the revolution speed of an engine, and a controller for receiving signals detected by those sensors, calculating and outputting a delivery displacement command value for the fan hydraulic pump depending on the cooling water temperature, the working oil temperature and the engine revolution speed, and continuously controlling the rotation speed of the cooling fan by the variable-displacement fan hydraulic pump (see, e.g., Patent Document 1).
Patent Document 1: JP, A 2001-182535